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United States Patent |
5,168,083
|
Matthews
,   et al.
|
December 1, 1992
|
High opacity defined kaolin product and method of producing same
Abstract
A high opacity, defined kaolin clay product having a relatively narrow
particle size distribution, low colloidal particle content and an average
particle surface area of less than about 30 square meters per gram is
prepared by defining an aqueous kaolin clay slurry via controlled
centrifugation to remove a substantial portion of the colloidal particles
therein. Prior to being subjected to centrifugation, the aqueous kaolin
clay slurry is prepared so as to improve the defining process achieved via
centrifugation by first subjecting the aqueous kaolin clay suspension to
scrub grinding so as to break up agglomerates into individual kaolin clay
particles without substantial delamination of the kaolin clay particles.
Thence, the mechanically dispersed kaolin clay suspension is dispersed to
its optimum level by the addition of a chemical dispersant, most
advantageously sodium hexamethaphosphate, and dilution water is admixed
into the aqueous kaolin clay suspension to reduce the solids content
thereof to a level less than at least about 18% solids by weight, and
preferably to about 5% to about 15% by weight, prior to centrifugration.
Inventors:
|
Matthews; Kirt L. (Macon, GA);
Miller; Bernard A. (Macon, GA)
|
Assignee:
|
Georgia Kaolin Company, Inc. (Elizabeth, NJ)
|
Appl. No.:
|
521204 |
Filed:
|
May 9, 1990 |
Current U.S. Class: |
501/146; 106/416; 106/486; 501/148; 501/149 |
Intern'l Class: |
C04B 033/04 |
Field of Search: |
106/486,416
501/144,145,146,147,148,149
|
References Cited
U.S. Patent Documents
2992936 | Jul., 1961 | Rowland | 106/486.
|
3058671 | Oct., 1962 | Billue | 501/145.
|
3343973 | Sep., 1967 | Billue | 501/145.
|
4018673 | Apr., 1977 | Hughes | 106/486.
|
4075030 | Feb., 1978 | Bundy et al. | 106/416.
|
4241142 | Dec., 1980 | Kaliski et al. | 501/145.
|
4650521 | Mar., 1987 | Koppelman | 106/486.
|
4888315 | Dec., 1989 | Bowman et al. | 501/144.
|
4916094 | Apr., 1990 | Salinas | 501/148.
|
4943324 | Jul., 1990 | Bundy et al. | 501/145.
|
Primary Examiner: Dixon, Jr.; William R.
Assistant Examiner: Marcantoni; Paul
Attorney, Agent or Firm: Klauber & Jackson
Claims
We claim:
1. A process for preparing a high opacity kaolin product having a
relatively narrow particle size distribution and low content of colloidal
particles, said process comprising the steps of:
a. mechanically dispersing a kaolin clay in water to form an aqueous kaolin
suspension of substantially unaggregated kaolin particles;
b. admixing a chemical dispersing agent with said aqueous kaolin suspension
produced in step (a), the chemical dispersing agent being added in an
amount necessary to ensure the formation of an optimally dispersed and
unaggregated aqueous kaolin suspension;
c. adding dilution water to the optimally dispersed aqueous kaolin
suspension in an amount sufficient to reduce the solids content of the
optimally dispersed aqueous kaolin suspension to from about 5 to 15%
solids by weight;
d. subjecting the low solids content optimally dispersed aqueous kaolin
suspension to a first centrifugation so as to fractionate said aqueous
kaolin suspension into an overflow slurry containing a substantial portion
of colloidal particle size material therein and a first underflow slurry
containing a relatively small portion of colloidal particle size material
therein;
e. rediluting said first underflow slurry with water to from about 5 to 15%
solids and subjecting the rediluted slurry to a second centrifugation so
as to fractionate said rediluted slurry into an overflow slurry containing
a substantial portion of colloidal particle size material therein and a
second underflow slurry containing a relatively small portion of colloidal
particle size material therein;
f. controlling the centrifugation steps so that said second underflow
slurry of kaolin particles has an average particle surface of less than
about 30 square meters per gram as measured by the methylene blue spot
test procedure, the low solids content aqueous suspension being subjected
to said centrifugation steps being maintained at a temperature of at least
about 100.degree. F.; and
g. collecting the second underflow slurry from said step (f) as product.
2. A process as recited in claim 1 further comprising maintaining the low
solids content aqueous suspension being subjected to centrifugation at a
temperature ranging from about 100.degree. F. to about 125.degree. F.
3. A process as recited in claim 1 further comprising controlling the
particle content of the overflow slurries to a solids level of less than
about 10% solids by weight.
4. A process as recited in claim 3 further comprising controlling the
particle content of the overflow slurries to a solids level ranging from
about 5% to about 10% solids by weight.
5. A process as recited in claim 1 further comprising treating said
underflow slurry from step (g) with a water soluble amine and aluminum
sulfate.
6. A process as recited in claim 5 wherein the step of treating said
underflow kaolin slurry with a water soluble amine and aluminum sulfate
comprises the steps of:
a. admixing hexamethylenediamine into said underflow kaolin slurry;
b. adjusting the pH of said admixture to a level ranging from about 2.6 to
about 2.8 by adding an acid thereto;
c. subjecting the kaolin particles in said acidified admixture to a
leaching treatment; and
d. admixing aluminum sulfate into said acidified admixture.
7. A process as recited in claim 6 wherein hexamethylenediamine is admixed
into said underflow kaolin slurry in an amount ranging from about 0.5 to
about 2.5 pounds per ton of dry clay.
8. A process as recited in claim 6 wherein aluminum sulfate is admixed into
said acidified mixture in an amount ranging from about 10 to about 20
pounds per ton of dry clay.
9. A method in accordance with claim 1 wherein centrifugation steps (d) and
(e) are conducted in separate centrifuges.
10. A method in accordance with claim 1, wherein step (d) and (e) are
conducted in a single centrifuge to which said rediluted slurry in step
(e) is internally recycled.
11. A method in accordance with claim 10, wherein said steps (d) and (e)
are carried out in an internal recycle disc-nozzle centrifuge.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a defined kaolin composition exhibiting
high opacity and to a method of producing the kaolin composition by
controllably defining a kaolin clay. More specifically, the present
invention relates to a beneficiated kaolin product having improved
opacifying efficiency thereby rendering the product functional as a high
performance paper coating or filler, and to a method of producing the
beneficiated kaolin product by controllably defining and otherwise
treating a base kaolin clay so as to remove a substantial portion of
colloidal particles therein.
It is well known in the paper industry, that a wide variety of pigments,
such as titanium dioxide, calcium carbonate, talc, synthetic silicates,
and clays such as bentonite and kaolin, are suitable for use as paper
fillers and/or coatings. Kaolin, a naturally occurring hydrated aluminum
silicate, is presently the most widely utilized and is available in a
range of particle sizes and brightnesses, as well as being either
delaminated or non-delaminated. Hydrated kaolin is white in color, has a
fine particle size, is relatively chemically inert, and makes an ideal low
cost paper filler. Although calcined (anhydrous) kaolin is also available
for use as a paper filler and can impart greater opacity to paper than the
hydrated kaolin, it has the serious disadvantage of being more abrasive.
Prior art kaolin paper fillers and coatings are typically produced by a
beneficiation process which typically consists of fractionating in a
continuous centrifuge to remove oversize material followed by leaching to
remove iron-based colored compounds. In the leaching process the kaolin is
acidified with H.sub.2 SO.sub.4 to a pH of 3.0 to solubilize the iron.
Sodium hydrosulfite is then added to reduce the iron to a more soluble
ferrous form which is removed during the dewatering process. The
flocculated clay, generally at approximately 30% solids by weight, is then
filtered, such as by dewatering on a rotary vacuum filter to a solids
level approximately 60% by weight. The filter cake is then either dried or
redispersed with additional dry clay if it is to be sold as approximately
70% by weight solids slurry. To produce high brightness products, i.e., a
product having a brightness index greater than 90, impurities may be
removed from the kaolin clay by further processing the kaolin clay through
flotation or magnetic separation. To produce a delaminated product, the
coarse fraction from the initial centrifugation is ground in sand grinders
to shear the stacks of platelets normally found in kaolin and thereby
produce individual particles having an equivalent spherical diameter less
than 2 microns.
It is well appreciated in the art that kaolin clay pigments must have
certain rheological and optical properties to be suitable for use in paper
manufacture as paper coatings or paper fillers. The kaolin clay pigment
must be available as a high solids suspension typically having a clay
solids content of about 50% to about 70% by weight, but still exhibiting a
viscosity low enough to permit efficient and economical pumping,
mixability with other filler or coating components, and application to the
paper. Additionally, it is of utmost importance that the kaolin pigment
exhibit certain optical properties, namely high brightness, high gloss and
high opacity.
The influence of particle size distribution upon the optical properties of
kaolin pigments has long been appreciated in the art. For example, in
commonly assigned U.S. Pat. No. 2,992,936, Rowland discloses that a kaolin
clay product having the following particle size distribution (in terms of
equivalent spherical diameter, e.s.d.) will consistently show improved
brightness, gloss and opacity when used as a paper coating clay:
99-100% by wt. less than 5 microns e.s.d.
98-100% by wt. less than 4 microns e.s.d.
88-100% by wt. less than 1.7 microns e.s.d.
85-97% by wt. less than 1.5 microns e.s.d.
70-84% by wt. less than 1.0 micron e.s.d.
25-37% by wt. less than 0.5 micron e.s.d.
10-15% by wt. less than 0.3 micron e.s.d.
Rowland further discloses producing such a controlled particle size kaolin
product by first degritting a kaolin clay slurry, thence passing the
degritted kaolin clay slurry at 21% solids by weight through a Sharples
centrifuge at 400 cc per minute at 6300 r.p.m. and then recentrifuging the
overflow effluent at the same rate and r.p.m. The final overflow effluent
represented a cut taken off the fine end of the degritted clay slurry and
amounted to 22% by weight of the degritted clay slurry. The degritted clay
slurry remaining after removal of this 22% fine cut, i.e., the combined
underflows from the two centrifugation steps, was reslurried to about 20%
solids with 0.15% sodium hexametaphosphate and allowed to settle by
gravity through a 1 inch slip depth. The sedimented coarse clay, which
amounted to about 48% of the degritted clay slurry, was discarded leaving
about 30% by weight of the original degritted kaolin clay slurry as an
intermediate product to be subjected to further treatment via bleaching,
filtering and drying to yield a commercial coating clay product.
In a paper entitled "Chemically Induced Kaolin Floc Structures for Improved
Paper Coating", presented at the 1983 TAPPI Coating Conference, W. H.
Bundy et al. disclosed an improved high bulking paper coating pigment,
referred to as 1089, which comprises a chemically modified kaolin produced
by the Georgia Kaolin Company, Inc. and marketed under the trade name
Astra-Lite. Structures of optimum functionality are said to be derived by
chemically treating a base kaolin clay having a particle size distribution
wherein from about 80% to 93% by weight of the kaolin particles are less
than 2 microns e.s.d. to selectively flocculate a portion of the submicron
fines therein thereby aggregating a portion of these fines on the surface
of larger kaolin platelets and effectively inactivating a large portion of
colloidal particles. Such a chemically modified kaolin coating pigment
derived from a base kaolin wherein 92% by weight particles under 2 microns
is presented by Bundy et al. as having a particle size distribution as
follows:
99% by wt. less than 5 microns e.s.d.
97% by wt. less than 3 microns e.s.d.
90.5% by wt. less than 2 microns e.s.d.
65.5% by wt. less than 1 micron e.s.d
31.5% by wt. less than 0.5 micron e.s.d.
12.5% by wt. less than 0.3 micron e.s.d.
5.5% by wt. less than 0.17 micron e.s.d.
Such a chemically flocculated kaolin coating pigment may be produced, for
example, as disclosed in U.S. Pat. Nos. 4,075,030; 4,076,548 or 4,078,941,
by selectively flocculating a base kaolin clay with the addition of either
a low molecular weight (less than 1,000,000) organic flocculent such as a
polyfunctional amine, e.g., ethylene diamine or hexamethylene diamine, or
long carbon chain amine, with or without citric acid and, optionally, in
the presence of fine mica below 150 mesh in size.
There is disclosed in U.S. Pat. No. 4,738,726, an opacifying pigment
composition suitable for use as a paper filler or coating which consists
essentially of particles of hydrous kaolin clay flocculated with a
controlled minor amount of a cationic polyelectrolytic flocculent, e.g., a
quaternary ammonium polymer salt or a diallyl ammonium polymer salt. The
base kaolin clay is selected to have a particle size distribution prior to
flocculation wherein less than 35% by weight are finer the 0.3 microns,
i.e., colloidal.
SUMMARY OF THE INVENTION
The process of the present invention provides a method of preparing a high
opacity, defined kaolin clay product having a relatively narrow particle
size distribution, low colloidal particle content and an average particle
surface area of less than about 30 square meters per gram. In accordance
with the process of the present invention, an aqueous kaolin clay slurry
is defined via controlled centrifugation to remove a substantial portion
of the colloidal particles therein thereby narrowing the particle size
distribution of the kaolin particles in the suspension whereby the
opacifying properties of the kaolin clay suspension are improved.
Prior to being subjected to centrifugation, the aqueous kaolin clay slurry
is prepared so as to improve the defining process achieved via
centrifugation by first dispersing the aqueous kaolin clay suspension and
then subjecting the dispersed suspension to scrub grinding so as to break
up agglomerates into individual kaolin clay particles without delaminating
the kaolin clay particles. Thence, the mechanically dispersed kaolin clay
suspension is dispersed to its optimum level by the addition of a chemical
dispersant, most advantageously sodium hexametaphosphate. After dispersing
the aqueous kaolin clay suspension, dilution water is admixed into the
aqueous kaolin clay suspension to reduce the solids content thereof to a
level less than at least about 18% solids by weight, and preferably to
about 5% to about 15% by weight, prior to centrifugation. Additionally,
the subsequent defining process may be further improved by preheating the
low solids aqueous kaolin clay suspension to a temperature of at least
100.degree. F. and preferably between 100.degree. F. and 125.degree. F.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a process flow diagram illustrating a preferred embodiment of the
process of the present invention;
FIG. 2 is a schematic diagram illustrating a method of defining an aqueous
kaolin clay suspension in accordance with the present invention using a
pair of centrifuges and equipped with underflow recycle; and
FIG. 3 is a schematic diagram illustrating a method of defining an aqueous
kaolin clay suspension in accordance with the present invention using a
single centrifuge equipped with underflow recycle.
DESCRIPTION OF A PREFERRED EMBODIMENT
Referring now to FIG. 1, there is depicted schematically therein a flow
diagram illustrating a preferred embodiment of the process of the present
invention wherein a kaolin clay is defined via subjecting a low solids,
optimally dispersed aqueous kaolin dispersion to centrifugation controlled
so as to produce an underflow slurry of kaolin particles having an average
particle surface area of less than about 30 square meters per gram.
All surface areas herein referred to are determined by a slightly modified
version of the standard methylene blue spot test procedure outlined by M.
J. Nevins and D. J. Weintritt in an article entitled "Determination of
Cation Exchange Capacity by Methylene Blue Adsorption", published in the
American Ceramics Society Bulletin, Volume 46, pages 587-592, 1967. In
accordance with this procedure, a one gram sample of powdered kaolin clay
prepared by spray drying and pulverizing the underflow kaolin slurry
produced after defining via controlled centrifugation was weighed into a
100 ml. beaker. Ten milliliters of deionized water was then added and the
aqueous clay suspension in the beaker continuously agitated with a
magnetic stirrer and stirring bar while methylene blue stock solution was
added in 0.5 milliliter increments. Approximately one minute after each
addition of methylene blue, a drop of liquid was removed from the beaker
with a glass rod and deposited on a piece of Whatman #50 filter paper. The
addition of methylene blue ceased when the dye first appeared as a blue
ring surrounding the dyed solids formed on the filter paper, the blue ring
indicating unabsorbed dye. The suspension was then mixed for an additional
two minutes after the initial appearance of the blue ring to assure
maximum adsorption of the dye by the clay solids in the suspension. An
additional drop was then removed from the beaker and placed on the filter
paper. If the blue ring again appeared, the test was considered completed.
If not, the addition on methylene blue stock was continued in 0.5
milliliter increments until a stable blue ring was obtained.
After achievement of a stable blue ring, the average surface area of the
kaolin particles was determined using the following formula:
##EQU1##
where: V.sub.MB =milliliters of methylene blue solution added
(meq/ml).sub.MB =milliequivalents of methylene blue per milliliter of
solution
(g.sub.DC)=dry weight of the clay sample in grams
In the preferred embodiment of the present invention as illustrated in FIG.
1, the crude kaolin clay 11 to be processed is blunged with water and
dispersant in a conventional manner in a commercially available blunging
apparatus 10 to produce an aqueous slurry of crude kaolin clay 13
typically having a solids content ranging from about 20% to 70% by weight
solids. This aqueous crude kaolin clay slurry 13 is next degritted, as in
conventional practice, to remove substantially all particles in excess of
44 microns (i.e., +325 mesh) equivalent spherical diameter (e.s.d.).
Advantageously, the degritting of the crude kaolin slurry is carried out
by first passing the crude kaolin slurry 13 through a screening device 12,
such as a sand box, to remove the coarsest material in the slurry and
thence fractionating the screened kaolin clay slurry 15 on a Bird
centrifuge 14 to separate a coarse fraction 17 comprising particles
greater than about 5 microns e.s.d. therefrom, which coarse fraction is
discarded. The discarded coarse fraction 17 typically constitutes about
50% of the original crude clay. The recovered fraction 19 of the degritted
clay slurry is thence passed through a conventional magnetic separator 16
to remove a substantial portion of the iron impurities therein and produce
a relatively low-iron content, degritted clay slurry 21 for further
processing.
In accordance with the present invention, the aqueous kaolin clay slurry 21
is further processed to produce the defined clay product of the present
invention by first scrub grinding the kaolin clay slurry to mechanically
disperse the kaolin clay to provide an aqueous suspension of unaggregated
kaolin clay particles. Thence a chemical dispersing agent is admixed with
this aqueous kaolin clay suspension to ensure optimal dispersion of the
kaolin particles in the suspension and thereafter dilution water is added
to the optimally dispersed aqueous kaolin clay suspension to reduce the
solids content of the suspension to an amount less than about 18% solids
by weight. This low solids content optimally dispersed aqueous kaolin clay
suspension is subjected to centrifugation so as to fractionate the kaolin
clay suspension into an overflow slurry containing a substantial amount of
the colloidal particle size material in the optimally dispersed suspension
fed to the centrifuge and an underflow slurry containing a substantially
reduced content of colloidal particle size material relative to the
optimally dispersed suspension fed to the centrifuge and having an average
particle size surface area of less than about 30 square meters per gram as
measured by the methylene blue spot test procedure.
The mechanical dispersing of the aqueous kaolin clay suspension 21 is
advantageously carried out by scrub grinding, that is, by passing the
aqueous kaolin clay suspension 21 through a wet media grinder 20 wherein
the aqueous kaolin clay suspension 21 is agitated in the presence of a
grinding media, typically plus 325 mesh (i.e., greater than about 44
micron diameter) sand. The aqueous kaolin clay particles are subjected in
the wet media grinder 20 to a scrubbing action which is sufficient to
break up agglomerates of clay particles into individual particles, but is
insufficient to separate the clay platelets making up the individual
particles. That is, the scrub grinding does not exert enough energy upon
the individual particles to delaminate the kaolin particles.
After screening in a conventional manner through screening apparatus 22 to
remove any grinding media, a chemical dispersant 5 is admixed into this
mechanically dispersed aqueous suspension 23 of deagglomerated,
undelaminated kaolin clay particles to further disperse the aqueous kaolin
clay suspension 23 to its optimum level of dispersion. Optimum dispersion
is achieved when the Brookfield viscosity of the aqueous kaolin clay
suspension reaches its minimum value. To ensure proper dispersion, the
chemical dispersant is admixed into the aqueous kaolin clay suspension in
an amount that results in the Brookfield viscosity of the aqueous kaolin
clay suspension reaching its minimum viscosity.
The chemical dispersant 5 admixed into the aqueous kaolin clay suspension
23 most advantageously comprises sodium hexametaphosphate, although other
known dispersants may also be employed. Sodium hexametaphosphate is
preferred since it has proven to be more effective in dispersing the fine
particle size material in the aqueous kaolin clay suspension 23. It is
desired to optimally disperse the fine particle size material so as to
improve the subsequent defining of the clay suspension. Typically, the
amount of sodium hexametaphosphate to be added to ensure proper dispersion
ranges from 0.5 to 5.0 pounds of sodium hexametaphosphate per ton of dry
clay.
After addition of the chemical dispersant 5, the optimally dispersed
aqueous kaolin clay suspension 23 is diluted by the addition of dilution
water 7 prior to subjecting the suspension to centrifugation to reduce the
solids content of the aqueous kaolin clay suspension to a solids content
less than about 18% by weight and preferably in the range of about 5% to
15% by weight. It has been found that centrifugation at such a low solids
content substantially improves defining as compared to centrifugation at
the higher solids levels in excess of 20% by weight typically employed in
conventional fractionation via centrifugation.
After addition of the water 7, the low solids aqueous kaolin clay slurry 25
may be passed directly to centrifuge means 30, but preferably is first
passed through heat exchanger 28 wherein the aqueous kaolin clay
suspension 25 is passed in indirect heat exchange relationship with a
heating medium, for example hot water, to preheat the kaolin clay
suspension 25 prior to centrifugation to a temperature of at least about
100.degree. F. (37.8.degree. C.). It has been found that preheating the
kaolin clay suspension results in a cleaner separation during
centrifugation thereby producing a more defined product. The colder the
suspension fed to the centrifuge means 30, the more fines present in the
underflow product suspension from the centrifuge means 30 at any given
defining level. The temperature to which the feed suspension to the
centrifuge means 30 may be preheated is limited by resultant boiling of
the underflow from the centrifuge means 30. That is, the temperature of
the feed suspension must be kept low enough to ensure that the underflow
suspension does not boil. Preferably, the aqueous kaolin clay suspension
25 is preheated to a temperature ranging from about 100.degree. F. to
about 125.degree. F. (37.8.degree. C. to 51.7.degree. C.) prior to
centrifugation.
In the centrifuge means 30 the aqueous kaolin clay feed suspension 25 is
subjected to centrifugation so as to fractionate the feed suspension 25
into an overflow suspension 33 and an underflow suspension 35. The
overflow suspension comprises the finer cut and contains a substantial
portion of the colloidal material originally contained in the feed
suspension 25. Conversely, the underflow suspension 35 comprises the
coarser cut and contains a colloidal particle size content substantially
lower than the colloidal solids content in the feed suspension 25. In this
manner, the aqueous kaolin clay feed suspension 25 is defined to yield as
the underflow an aqueous kaolin clay suspension 35 which because of the
removal of a substantial amount of the finer material therein, in
particular the colloidal solids, has a much narrower particle size
distribution, and consequently exhibits better opacifying ability, than
the aqueous kaolin clay feed suspension 25.
As used herein, the term "defining" refers to the operation of separating
and recovering a portion of the kaolin clay having a reduced content of
finer particle size kaolin material, in particular colloidal particle size
material, that is kaolin particles having a particle size less than about
0.3 micron equivalent spherical diameter. Defining level or percentage as
used herein refers to the amount of dry clay, on a weight percent basis,
removed from the kaolin clay feed suspension in the overflow suspension.
For example, a defining level of 35% means that 35% by weight of the dry
clay in the aqueous kaolin clay suspension 35 has been removed with the
overflow.
In practice, the centrifugation of the low solids aqueous kaolin clay
suspension 25 is controlled so as to produce as an underflow product
suspension 35 of kaolin clay particles having a relatively narrow particle
size distribution and exhibiting an average particle surface area of less
than about 30 square meters per gram as measured by the methylene blue
spot test procedure hereinbefore outlined. By controlling the solids level
in the overflow suspension 33 by recycle to less than about 10% solids by
weight, it has been possible to consistently produce a defined product
having the desired opacifying properties at defining levels ranging from
about 15% to about 50% by weight for Createous kaolin clays. Such a
defined product will advantageously have a particle size distribution as
follows:
99-100% by wt. less than 5 microns e.s.d.
87-93% by wt. less than 2 microns e.s.d.
65-75% by wt. less than 1 microns e.s.d.
20-35% by wt. less than 0.5 microns e.s.d.
5-15% by wt. less than 0.3 microns e.s.d.
The underflow aqueous kaolin clay suspension 35 from the centrifugation
step is collected as the desired product and further processed according
to its intended use. For example, if the product is to be used as a paper
coating clay, the underflow aqueous kaolin clay suspension 35 from the
centrifugation step, which is typically at a solids content of about 40%
to about 50% by weight, is first diluted with water to a solids content of
about 15% to about 25% solids by weight, treated with sulfuric acid to
reduce its pH to a level between 2.5 and 3.0, and leached in a
conventional manner by adding thereto an aqueous solution of a reducing
agent, for example sodium dithionite solution at a treatment level of 2 to
6 pounds of sodium dithionite per ton of dry clay. After leaching to
improve brightness, the pH of the leached aqueous kaolin clay suspension
is adjusted to 3.0, filtered on a rotary vacuum filter, rinsed and
reblunged. A portion of this suspension is then spray dried and the spray
dried product pulverized and remixed with the remainder of the suspension
to produce the desired coating clay product at a solids level about 65%
solids by weight.
If, however, the underflow aqueous kaolin clay suspension 35 from the
centrifugation step is to be used for paper filling applications, the
underflow aqueous kaolin clay suspension 35 is again first diluted with
water to a solids content of about 15% to about 25% solids by weight, but
thence prior to leaching is treated with an amine, typically
hexamethylenediamine at a treatment level of about 0.5 to about 2.5 pounds
hexamethylenediamine per ton of dry clay, before adjusting the pH of the
treated suspension to a level between 2.5 and 3.0, thence preferably
adding aluminum sulfate (alum) to the amine treated suspension, typically
at a rate of about 10 to about 20 pounds of alum per ton of dry clay,
after leaching and further processing the aqueous clay suspension as
hereinbefore described with respect to the production of a coating clay
product.
Referring now to FIG. 2, there is depicted schematically therein one
embodiment of carrying out centrifugation in accordance with the present
invention by passing the prepared aqueous kaolin clay feed suspension 25
through a pair of centrifuge means 30a and 30b. Advantageously, centrifuge
means 30a and 30b each comprise disc-nozzle type centrifuges equipped for
internal recycle. Such internal recycle disc-nozzle type centrifuges are
commercially available from Dorr-Oliver Incorporated of Stamford, Conn.,
and Alfa-Laval Inc. of Fort Lee, N.J.
As depicted in FIG. 2, the low solids content aqueous kaolin clay feed
suspension 25 is fed into the first centrifuge means 30a and fractionated
therein into an overflow suspension 33a comprising a low solids slurry of
finer kaolin particles and containing a substantial portion of the
colloidal material originally contained in the feed suspension 25 and an
underflow suspension 27 comprising a higher solids slurry containing a
substantially lower content of colloidal solids than the feed suspension
25.
The underflow slurry 27 is collected and its solids content reduced by the
addition of dilution water 29 back to a level less than about 18% solids
by weight and preferably in the range of 5% to 15% solids by weight prior
to passing the diluted underflow slurry 27 into the second centrifuge
means 30b. In the second centrifuge means 30b, the diluted underflow
slurry 27 from the first centrifuge means 30a is subjected to further
fractionation into an overflow suspension 33b and a product underflow
suspension 35. The overflow suspension 33b comprises a low solids slurry
of finer kaolin particles and contains a substantial portion of the
colloidal material remaining in the underflow slurry 27 produced in the
fractionation in the first centrifuge means 30a. The underflow suspension
35 comprises a higher solids slurry containing a further reduced content
of colloidal material. The underflow slurry 35 from the second centrifuge
means 30b has a colloidal content which is somewhat lower than the
colloidal content of the underflow slurry 27 from the first centrifuge
means 30a and much less than the colloidal content in the original low
solids aqueous kaolin clay feed suspension 25 fed to the centrifuge means
30a. The underflow kaolin suspension 35 from the second centrifuge means
30b is collected as a desired product and further processed as
hereinbefore indicated depending upon its intended use. In controlling the
fractionation processes carried out in the centrifuge means 30a and 30b,
the solids level in the overflow suspensions 33a and 33b are controlled to
less than about 10% solids by weight.
Referring now to FIG. 3, there is depicted schematically therein another
embodiment of carrying out centrifugation in accordance with the present
invention by passing the prepared aqueous kaolin slurry feed suspension 25
through a single centrifuge means 30c which advantageously comprises an
internal recycle disc-nozzle type centrifuge. Such internal recycle
disc-nozzle type centrifuges are commercially available from Dorr-Oliver
Incorporated of Stamford, Conn., and Alfa-Laval Inc. of Fort Lee, N.J.
As depicted in FIG. 3, the low solids content aqueous kaolin clay feed
suspension 25 is fed into the internal recycle centrifuge means 30c and
fractionated therein into an overflow suspension 33 comprising a low
solids slurry of finer kaolin particles and containing a substantial
portion of the colloidal material originally contained in the feed
suspension 25 and an underflow suspension 35 comprising the coarser cut
and containing a reduced level of colloidal particle size material, which
reduced level is substantially lower than the colloidal solids content in
the feed suspension 25. The underflow aqueous kaolin clay suspension 35
from the internal recycle disc-nozzle centrifuge means 30c is collected as
a desired product and further processed according to its intended use as
hereinbefore outlined.
In such an internal recycle disc-nozzle centrifuges, a selectively
controlled portion of the underflow suspension discharged from the
centrifuge means is internally recycled back through the centrifuge means.
By internally recycled, it is meant that the recycled portion of the
underflow suspension is not admixed with the feed suspension being
supplied to the centrifuge for initial fractionation, but rather is
separately fed back to the centrifuge and directed internally through the
centrifuge means separately from the feed suspension so that it does not
undergo fractionation with the feed suspension, but rather is admixed with
the underflow slurry produced upon fractionation of the feed suspension
immediately upstream of the nozzle passages through which the newly formed
underflow suspension and the recycled underflow slurry admixed therewith
exit from the separation chamber within the centrifuge means. Internal
recycle is to be distinguished from external recirculation wherein the
recycled portion of the underflow suspension is admixed with the feed
suspension upstream of the centrifuge means so as to admix therewith and
undergo further fractionation therewith when passing through the
centrifuge means.
Again, by controlling the solids content in the overflow slurry to a level
less than about 10% solids by weight, it has been found that the
centrifugation of a low solids aqueous kaolin suspension prepared for
centrifugation in accordance with the present invention may be
consistently controlled to produce a defined product having a relatively
narrow particle size distribution. In controlling the centrifugation
process as carried out in the present invention, the average particle
surface area is monitored by measuring at periodic intervals by the
methylene blue spot test as hereinbefore described to ensure that the
product underflow suspension has an average particle size that is less
than about 30 meters squared per gram as determined by the methylene blue
spot test. If the measured average particle size surface area exceeds 30
meters squared per gram, it is an indication that the colloidal solids
content in the underflow slurry is increasing beyond desired levels and it
is necessary to increase the amount of underflow slurry internally
recycled through the centrifuge means which in turn increases the overflow
suspension which in turn increases the amount of fines removed and
consequently decreases the amount of fines in the underflow suspension
discharged from the centrifuge means. As the amount of fines in the
underflow suspension decreases, the average particle size in meters
squared per gram will also decrease thereby returning the underflow
suspension into the desired range. It has been found that when the surface
area exceeds 30 meters squared per gram, the opacity of a filled sheet
containing that product is not improved over the opacity of sheets formed
with commercial fillers.
The effectiveness of method of the present invention in producing a kaolin
product of improved opacity will now be illustrated through the following
examples.
In the examples to be presented, all opacity measurements were obtained on
handsheets incorporating the respective defined products of Examples I
through IV as fillers at a handsheet filler content of 10%. The opacity
measurements were obtained using TAPPI standard test methods. The opacity
measurements associated with each of the defined products of the following
examples was compared with the measured opacity for a handsheet
incorporating Astra-Fil.RTM. 90, a commercially available high performance
delaminated filler kaolin clay manufactured by Georgia Kaolin Company,
Inc. of Union, N.J., to determine the opacity improvement, that is the
increase in points over the opacity associated with Astra-Fil.RTM. 90, for
each defined product.
EXAMPLE I
A crude Cretaceous kaolin clay was blunged in water to form an aqueous
kaolin suspension which was degritted, subjected to magnetic separation
and mechanically dispersed via scrub grinding as hereinbefore described.
This aqueous kaolin suspension was then optimally dispersed via the
addition thereto of sodium hexametaphosphate solution to form a dispersed
aqueous kaolin suspension at a solids content of 26% solids by weight.
This dispersed aqueous kaolin suspension was then subjected to
centrifugation at a solids level of 26% by weight (i.e., without dilution)
to define this dispersed aqueous kaolin feed suspension at a defining
level of 38.9%, that is 38.9% by dry weight of the kaolin clay in the feed
suspension was removed during centrifugation. Centrifugation was carried
out in two steps by first passing the feed suspension through an internal
recycle disc-nozzle centrifuge to fractionate the feed suspension into an
overflow fraction comprising the finer cut and an underflow fraction
comprising the coarser cut. The overflow fraction was then again subjected
to a centrifugation to fractionate this overflow suspension from the first
centrifugation into an overflow suspension comprising the finer cut and an
underflow suspension comprising the coarser cut. The overflow suspension
from the second centrifugation step, which included 38.9% by weight (dry
clay basis) of the clay in the original feed suspension, was discarded.
The underflows from the first and second centrifugation steps were
collected and combined as the desired defined product. The combined
defined underflow suspensions were thence treated with
hexamethlyenediamine (HMDA) at a treatment level of 4 pounds HMDA per ton
of dry clay, acid flocced to a pH between 2.6 and 2.8 via the addition of
sulfuric acid, leached, treated with aluminum sulfate at a treatment level
of 14 pounds Alum per ton of dry clay, filtered, rinsed, and slurried to a
solids content of about 65% by weight via spray drying and remixing as
hereinbefore described to yield a defined kaolin clay filler product. The
opacity impvovement exhibited by the filler product of this Example I was
determined by the procedure hereinbefore described to be +0.1 points.
EXAMPLE II
A crude Cretaceous kaolin clay was blunged in water to form an aqueous
kaolin suspension which was degritted, subjected to magnetic separation
and mechanically dispersed via scrub grinding as hereinbefore described.
This aqueous kaolin suspension was then optimally dispersed via the
addition thereto of sodium hexametaphosphate to form a dispersed aqueous
kaolin suspension at a solids content of 25.6% solids by weight. This
dispersed aqueous kaolin suspension was then subjected to centrifugation
at a solids level of 25.6% by weight (i.e., without dilution) to define
this dispersed aqueous kaolin feed suspension at a defining level of
31.8%. Centrifugation was carried out in two steps in the manner
illustrated in FIG. 2. That is, the feed suspension was first passed
through an internal recycle disc-nozzle centrifuge to fractionate the feed
suspension into an overflow fraction comprising the finer cut and an
underflow fraction comprising the courser cut. The overflow suspension was
then discarded and the underflow suspension was then again subjected to a
centrifugation to fractionate this underflow suspension from the first
centrifugation into an overflow slurry comprising the finer cut thereto
which was discarded and an underflow slurry comprising the courser cut
which was collected as the desired defined product. Together, the
discarded overflow suspensions from the first and second centrifugation
steps included therein 31.8% by weight (dry clay basis) of the clay in the
original feed suspension. The defined underflow suspension from the second
centrifugation was leached, filtered, rinsed, and slurried via spray
drying and remixing as hereinbefore described to yield a defined kaolin
clay product. The opacity improvement exhibited by the kaolin clay product
was determined by the procedure hereinbefore described to be +1.4 points.
EXAMPLE III
A crude Cretaceous kaolin clay was blunged in water to form an aqueous
kaolin suspension which was degritted, subjected to magnetic separation
and mechanically dispersed via scrub grinding as hereinbefore described.
This aqueous kaolin suspension was then optimally dispersed via the
addition thereto of sodium hexametaphosphate to form a dispersed aqueous
kaolin suspension at a solids content of 27.2% solids by weight. This
dispersed aqueous kaolin suspension was then subjected to centrifugation
at a solids level of 27.2% by weight (i.e., without dilution) to define
this dispersed aqueous kaolin feed suspension at a defining level of
38.2%. Centrifugation was carried out in a single step in the manner
illustrated in FIG. 3 using an internal recycle disc-nozzle centrifuge to
fractionate the feed suspension into an overflow fraction comprising the
finer cut and an underflow fraction comprising the courser cut. The
overflow suspension was then discarded and the underflow suspension was
leached, filtered, rinsed, and slurried via spray drying and remixing as
hereinbefore described to yield a defined kaolin clay product. The opacity
improvement exhibited by the kaolin clay product was determined by the
procedure hereinbefore described to be +0.3 points.
EXAMPLE IV
A portion of the defined underflow suspension from the second
centrifugation step of Example III was treated with HMDA at a treatment
level of 4 pounds HMDA per ton of dry clay, acid flocced to a pH between
2.6 and 2.8 via the addition of sulfuric acid, leached, treated with
aluminum sulfate at a treatment level of 14 pounds Alum per ton of dry
clay, filtered, rinsed, and slurried to a solids content of about 65%
solids by weight via spray drying and remixing as hereinbefore described
to yield a defined kaolin clay filler product. The opacity improvement
exhibited by the filler product of this Example III was determined by the
procedure hereinbefore described to be +1.3 points.
EXAMPLE V
A crude Cretaceous kaolin clay suspension was prepared, defined via
two-step centrifugation, and the resultant underflow suspension treated as
described in Example II with the exception that the dispersed aqueous
kaolin suspension was diluted by the addition of water thereto prior to
both centrifugation steps to a solids content of 7.1% by weight in
accordance with the present invention. This diluted aqueous kaolin
suspension was defined at a defining level of 39.7%. The opacity
improvement exhibited by the filler product of this Example IV was
determined by the procedure hereinbefore described to be +1.8 points.
EXAMPLE VI
A crude Cretaceous kaolin clay suspension was prepared, defined via
two-step centrifugation, and the resultant underflow suspension treated as
described in Example II with the exception that the dispersed aqueous
kaolin suspension was diluted by the addition of water thereto prior to
both centrifugation steps to a solids content of 8.0% by weight in
accordance with the present invention. This diluted aqueous kaolin
suspension was defined at a defining level of 36.8%. The opacity
improvement exhibited by the filler product of this Example IV was
determined by the procedure hereinbefore described to be +1.8 points.
EXAMPLE VII
A crude Cretaceous kaolin clay suspension was prepared, defined via
two-step centrifugation, and the resultant underflow suspension treated as
described in Example VI with the exception that the aqueous kaolin
suspension was not mechanically dispersed by scrub grinding prior to
centrifugation but rather only chemically dispersed via the addition of
sufficient amounts of sodium hexametaphosphate to ensure optimum
dispersion as hereinbefore described. This aqueous kaolin suspension was
defined at a defining level of 38.0%. The opacity improvement exhibited by
the filler product of this Example VI was determined by the procedure
hereinbefore described to be +0.7 points.
EXAMPLE VIII
A crude Cretaceous kaolin clay was blunged in water to form an aqueous
kaolin suspension which was degritted, subjected to magnetic separation
and mechanically dispersed via scrub grinding as hereinbefore described.
This aqueous kaolin suspension was then optimally dispersed via the
addition thereto of sodium hexametaphosphate to form a dispersed aqueous
kaolin suspension at a solids content of 27.2% solids by weight. This
dispersed aqueous kaolin suspension was diluted by the addition of water
thereto to a solids content of 14.3% by weight in accordance with the
present invention and them subjected to centrifugation at a solids level
of 14.3% to define this dispersed aqueous kaolin feed suspension at a
defining level of 34.0%. Centrifugation was carried out in a single step
using an internal recycle type disc-nozzle centrifuge in the manner
illustrated in FIG. 3. The overflow suspension from the centrifugation,
which comprised the finer cut and included 34% by weight (dry clay basis)
of the kaolin content of the feed suspension, was discarded. The underflow
suspension comprised the coarser cut and was collected as the desired
defined product. This underflow suspension was then treated with
hexamethylenediamine and aluminum sulfate and further processed as
described in Example IV to yield a defined kaolin clay filler product. The
opacity improvement exhibited by the filler product of this Example VIII
was determined by the procedure hereinbefore described to be +1.8 points.
The opacity improvements exhibited by each of the defined kaolin clay
products of Examples I through VIII are presented hereinbelow in Table I.
As is shown therein, the greatest opacity improvements (+1.8 points) were
exhibited by those products which were not only produced by defining at
low solids level in accordance with the present invention, but were also
prepared prior to defining by mechanically dispersing the aqueous kaolin
clay via scrub grinding to break up kaolin agglomerates prior to
chemically dispersing the suspension to minimum viscosity before defining.
TABLE I
______________________________________
Sample
Feed Mechanically
Defining
Treat-
Opacity
Name Solids Dispersed Level ed Over AF-90
______________________________________
I 26 Yes 38.9 Yes 0.1
II 25.6 Yes 31.8 Yes 1.4
III 27.2 Yes 38.2 No 0.3
IV 27.2 Yes 38.2 Yes 1.3
V 7.1 Yes 39.7 Yes 1.8
VI 8.0 Yes 36.8 Yes 1.8
VII 8.0 No 38.03 Yes 0.7
VIII 14.3 Yes 34.0 Yes 1.8
______________________________________
A comparison of the opacity improvements presented in Table I also confirms
that defining by the two-step process of the present invention (Examples
IV and V) wherein the underflow suspension, i.e., the coarse cut, from the
first centrifugation step is subjected to a second centrifugation step and
the overflow suspensions from both centrifugation steps, yields a defined
product which exhibits a substantially greater opacity improvement when
compared to a product defined at the same defining level but defined by
the typical prior art two-step centrifugation exemplified by Example I
wherein the overflow suspension, i.e., the finer cut, from the first
centrifugation step is subjected to a second centrifugation step and the
underflow suspensions from both the first and second centrifugation steps
combined to yield the defined product as described in U.S. Pat. No.
2,992,936.
EXAMPLE IX
A crude Cretaceous kaolin clay suspension was prepared as outlined in
Example VII and defined via single stage centrifugation using an internal
recycle type disc-nozzle centrifuge in the manner illustrated in FIG. III
at a feed solids level of 15% by weight and a defining level of 35%. The
underflow suspension, comprising the coarser cut, was collected and
divided into two portions, one portion of which was then treated with
hexamethylenediamine and aluminum sulfate and further processed as
described in Example III, while the other portion was not treated with
either hexamethylenediamine or aluminum sulfate, but rather merely acid
flocced, leached, filtered, rinsed and reslurried.
The treated and untreated products prepared as presented in Example VIII
where used in formulate paper coating compositions by admixing 100 parts
by weight of the sample product, 12 parts by weight of a latex binder, 6
parts by weight of a starch binder, and 1 part by weight of calcium
stearate. For comparison purposes, coating compositions were also prepared
via this formulation using a No. 1 coating clay and a No. 1 high
brightness kaolin. Each coating formulation was then applied to a 47
lb/3300 ft.sup.2 basestock paper at a coating weight of 7.1 lb/3300
ft.sup.2. The coated sheets were then calendered two nips at 140.degree.
and 200 psi pressure, before measurements of opacity, brightness, paper
gloss and print gloss were taken in accordance with standard TAPPI
methods, which measurements are reported in Table II.
TABLE II
______________________________________
Paper Print
Clay in Coating
Opacity Brightness
Gloss Gloss
______________________________________
No. 1 High Brightness
87.6 79.3 59 79
No. 1 Coating 87.7 78.2 58 79
Ex. VIII - Treated
88.8 80.7 69 88
Ex. VIII - Untreated
88.7 80.6 67 86
______________________________________
A comparison of the opacity, brightness, the paper gloss and print gloss of
the paper coatings prepared using the defined kaolin products of Example
VIII, both treated and untreated, with those of paper coatings prepared
using standard No. 1 coating clay or No. 1 high brightness clay clearly
confirms that a defined kaolin clay produced not only by defining at low
solids level in accordance with the present invention, but also prepared
prior to defining by mechanically dispersing the aqueous kaolin clay via
scrub grinding to break up kaolin agglomerates prior to chemically
dispersing the suspension to minimum viscosity before defining, will
exhibit superior performance as a coating clay with or without subsequent
surface treatment.
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